@sab8 I'm bipolar and think this is a very interesting ask. My kids are 3 & 7.
I'm pretty transparent with them. They know I go to a 'feelings doctor' (therapist) and she helps me keep my feelings from getting so big that they get me flooded and make me too angry or too sad. We're teaching mindfulness and self-regulation skills so this language is known.
I take 'meds' or 'medication' and the kids help me fill my weekly pill bottles. I need the meds because my brain doesn't work the same as most people. It gives me bigger feelings and makes it harder to control them. My brain lies to me and gives me thoughts that aren't true and I have to learn to tell those thoughts they're wrong!
I can't imaging having bipolar manifested as I do and keeping it from the kids. They're volatile because they're kids and get flooded so easily. But I'm volatile because my brain doesn't work the way most people's do, and the things I'm learning about myself will help them, and if they happen to be diagnosed later in life, they'll have good tools to get them started in self-regulation. They see me cry, a lot. They see me have anxiety attacks and it's impossible to totally hide my mental illness from people who live with me, but I do my best.
Any book describing mental illness should have some mention of the fact that it can be scary for the kids. Try not to be too condescending. They're small and young but not stupid.